In recent years, at an aim for the application of pixel circuits of liquid-crystal displays and organic electroluminescent displays, development is energetically made on semiconductor elements transparent to visible light which have thin-film transistors making use of transparent oxide semiconductors in active layers.
A thin-film transistor element currently in common use is shown in FIG. 1 as a sectional view. First, a gate metal is formed on an insulating substrate 1, and is then patterned in a desired shape to form a gate electrode 2. On this gate electrode, a gate insulating layer 3, a semiconductor film 4, a source electrode 5 and a drain electrode 6 are formed in this order, thus a thin-film transistor element is completed. In what is shown in FIG. 1, a protective layer 7 is further formed for the purpose of protecting the thin-film transistor element.
In such a thin-film transistor element, there are various points to which attention should be paid in order to achieve good transistor characteristics. For example, Japanese Patent Application Laid-open No. H06-045605 discloses a problem that any unevenness of an interface between the gate insulating layer and the semiconductor layer may cause a lowering of electron mobility. The unevenness of an interface between the gate insulating layer and the semiconductor layer is also considered to cause an increase in interface trap level, an increase in effective channel length, unevenness of layer thickness of the semiconductor layer, stepped-cut of the semiconductor layer, and so forth. That is, how the interface between the gate insulating layer and the semiconductor layer be kept smooth is one of important points in order to keep the electron mobility from lowering, to improve characteristics of thin-film transistors.
In thin-film transistors transparent to visible light, a solid solution of indium oxide and tin oxide (hereinafter termed “ITO”) is commonly used as a transparent electrode. ITO thin films are obtained in a polycrystalline or amorphous form depending on conditions for their formation. Compared with polycrystalline ITO thin films, amorphous ITO thin films have a better surface smoothness and have a superior readiness in fine processing by etching. On the other hand, the amorphous ITO thin films have an inferior electric conductivity, and hence the polycrystalline ITO thin films are commonly used for electrodes.
However, if the ITO thin film is formed as a transparent gate electrode under such conditions that it comes to have the polycrystalline form at the time its formation has been completed, a large unevenness reflecting the crystal habit of crystal grains may inevitably come about on the surface of the thin film obtained. Then, where a gate insulating layer laid over the thin film is amorphous, the gate insulating layer is affected by the underlying gate electrode to come into a layer having a greatly uneven surface.
As stated above, if a semiconductor layer is formed on such a gate insulating layer having a large surface unevenness, the interface between the gate insulating layer and the semiconductor layer becomes greatly uneven to inevitably cause deterioration of thin-film transistor characteristics. Thus, in a bottom gate type thin-film transistor in which the gate insulating layer, the semiconductor layer, the source electrode/drain electrode and the protective layer are formed in this order on the gate electrode, there has been a problem that any sufficient characteristics are not obtainable when the polycrystalline ITO thin film is used as the gate electrode.